If you're going to a Nats game, you can now catch a luxury charter bus from Clyde's in Ashburn or The Greene Turtle at Dulles Town Center for $18 round trip ($15 if you book early). Bus stations also in Reston, Centreville and Ballston. Direct, non-stop service from each location.
http://www.ballparkbus.com/
"How It Works" video on YouTube:
http://youtu.be/7cel6osANJI